Metallocene catalysis polystyrenes

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

LRP is a powerful tool for the synthesis of complex polymer architectures as was shown above. However, in some cases it is desirable to combine structures that are hardly or not at all accessible via radical polymerization techniques. In such cases it may be beneficial to combine LRP with another polymerization mechanism. Many examples have been reported so far. A few examples will be listed here. Polystyrene-6-pol3risobutylene-6-polystyrene was synthesized via a combination of living cationic polymerization and ATRP (98). Polyolefin Graft Copolymers (qv) were synthesized by first polymerizing alkoxyamine-substituted olefins via metallocene catalysis, and subsequent polymerization of vinyl monomers via... 

In syndiotactic or syntactic macromolecules, the substituents have alternate positions along the chain (Figure 20.1). The macromolecule consists of 100% racemo diads. Syndiotactic polystyrene (sPS), made by metallocene catalysis polymerization, is crystalline with a melting temperature of 161°C. 

Syndiotactic polystyrene is produced using metallocene catalysis and, unlike the atactic polymer, is highly crystalline with a of270°C. Most grades offered are glass-... 

In comparison with the extensive and fruitful investigations in the area of homogeneous syndiose-lective styrene polymerization, research in the field of isoselective polymerization is still trudging. Isotactic polystyrene, known for almost a half century, is best produced by heterogeneous Ziegler-Natta catalysis. Recently, the homogeneous isoselective polymerization of styrene by soluble metallocenes and nonmetallocene transition metal complexes has been gradually developed. 

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